This invention relates to phenol-formaldehyde-resorcinol resin compositions, to adhesives prepared therefrom, and to methods of making and using such resins and adhesives.
Many different types of phenol-formaldehyde-resorcinol resins, sometimes called PFR resins hereafter, have been known for almost 30 years. They have found wide acceptance, particularly within the forest product industry. Their excellent durability and property of curing to a thermoset state without external heat have made them especially useful in the manufacture of laminated wood products. They are also widely used in wood boat construction.
Those PFR adhesives available to date have been quite use specific. By this it is meant that an adhesive useful for radio frequency cured finger jointing or for general cold set laminating or for preheat laminating (where the surface area of the wood is heated before application of the adhesive and subsequent clamping) would not be suitable for another of those uses. A serious and expensive problem has been the lack of water washability of these adhesives. Normally when contacted with water these adhesives will curdle into a sticky, gooey mass that is then extremely difficult to remove from the surface to which they are adhering. This has necessitated the use of strong caustic solutions or organic solvents for cleanup of equipment.
These PFR resins have been made by mixing phenol and resorcinol in a suitable reaction vessel and then reacting formaldehyde with them. (U.S. Pat. No. 2,385,372 to Rhodes). PFR resins have also been made by reacting formaldehyde and phenol in one vessel and reacting formaldehyde with resorcinol in a separate vessel and then combining the two resulting reaction products. See U.S. Pat. Nos. 2,385,373 and 2,478,943 to Rhodes. It is also known to make a phenol-formaldehyde resin and then modify it with resorcinol to obtain PFR resins. One reason for this post modification by resorcinol is that the phenol-formaldehyde will become the backbone (i.e., the inner structure) of the final polymer and the resorcinol will be attached at reactive terminal positions. This is desirable because resorcinol is much more expensive than phenol but is also much more reactive. Thus, by this process the more costly resorcinol can be used most efficiently. U.S. Pat. Nos. 2,489,336 to Spahr et al, 2,614,096 to Spahr and Re. 26,881 to Kreibich et al teach generally these basic techniques.
In all of these prior patents the phenol-formaldehyde component has been a resole of moderate degree of polymerization.
U.S. Pat. No. 2,437,710 to Rhodes teaches a somewhat different approach. Among the many possible procedures which may be included under the teachings of this patent is the reaction of methylolated phenolic materials, such as o, o-dimethylol-p-cresol, with resorcinol in a nearly water free molten mass.
This invention is based on the discovery that by reacting methylolphenols with free formaldehyde and resorcinol, within certain molar ratios, an unexpectedly versatile resin product is obtained that may be further modified to form a variety of other useful resins. Among these, it has been discovered that when this particular resin is subjected to further advancement and mixed with a hardener the resulting resin composition produces an unusually versatile, durable and strong resin adhesive.